New invisibility cloak hides in the fog

Simple scheme uses cloudy environment to mask object

CLOUDED IN SECRECY A newly developed invisibility cloak (right) can hide objects in a hazy medium such as fog or frosted glass, as shown in an illustration. At right, the cloak wraps light around an object to fill in the gap that would otherwise appear as a dark shadow (left). The technique works because, unlike in air, light takes a long, pinball-like trip when traveling through such a medium (light shown as red streaks bouncing off blue particles in magnifying glass).

Want to disappear? A new invisibility cloak offers you the chance — provided you carry around a portable fog machine.

The cloak, described June 5 in Science, achieves the challenging feat of hiding an object from all angles and all colors of visible light, with the caveat that it works only in hazy environments including clouds, fog and frosted glass. “It’s a nice demonstration,” says Jason Valentine, a mechanical engineer at Vanderbilt University in Nashville. “It sacrifices functionality in one area to gain functionality in another” — namely, the disappearance of the object to the naked eye.

In 2006, physicists caught invisibility cloak fever after developing intricate synthetic structures called metamaterials, which bend light like no material found in nature. The hope was that these materials could coax light rays to curve around an object rather than strike it, making it imperceptible (SN: 7/15/06, p. 42).

But a key obstacle arose: To achieve invisibility, light taking a detour around a cloaked object would have to catch up to light traveling in a straight line. That’s challenging because light moves really fast through air — nearly at the speed of light in a vacuum, nature’s unbreakable speed limit. “Employing a cloak in the Harry Potter–type fashion is not doable now and maybe not doable ever,” Valentine says.

So Robert Schittny, a physicist at the Karlsruhe Institute of Technology in Germany, and colleagues decided to design a cloak that wouldn’t have to race against the speed of light. They realized that rendering objects invisible would be a lot easier in a medium such as fog. Instead of zipping through these environments, light bounces off the densely packed particles like a pinball and diffuses gradually, similar to the way an odor wafts across a room.

Taking advantage of this slowdown, the team designed a simple cloak made of common, inexpensive materials that completely hides an object from view, as long as it’s immersed in a hazy medium. For their experiment, Schittny and his colleagues chose a tank of water mixed with particles of commercial white wall paint to scatter and slow incoming light. The cloak consisted of a stainless steel cylinder or sphere coated with white paint and surrounded by a silicone shell doped with microparticles. These materials altered the speed and spread of rays of light as they bounced through the tank.

When a computer monitor next to the tank shined white light, the materials steered the light around the cloak and anything placed inside it, brightening the space behind the cloak that would otherwise appear dark. From the other side of the tank, the cloak was barely perceptible, causing just a subtle lightening of the water’s color; without the cloak, a shadowy profile of the object was easily visible.